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1. Field of the Invention (Technical Field)
The present invention relates to methods and apparatus for detection and quantification of heavy metals utilizing bismuth-coated electrodes, which electrodes may be coated with bismuth prior to use, or may be coated with bismuth in situ in a solution including a determined amount of bismuth together with the analyte.
2. Background Art
Note that the following discussion refers to a number of publications by author(s) and year of publication, and that due to recent publication dates certain publications are not to be considered as prior art vis-a-vis the present invention. Discussion of such publications herein is given for more complete background and is not to be construed as an admission that such publications are prior art for patentability determination purposes.
Electrochemical stripping analysis is a powerful electroanalytical technique for trace metal measurements (J. Wang, Stripping Analysis, VCH Publishers, Deerfield Beach, 1985; E. P. Achterberg, C. Braungradt, Anal. Chim. Acta 400(1999)381; M. Taillefert, G. W. Luther, D. B. Nuzzio, Electroanalysis 12(2000)401). A proper choice of the working electrode is crucial for the success of the stripping operation. Film and drop mercury electrodes have been traditionally used for achieving high sensitivity and reproducibility. Mercury electrodes can be used to detect up to four to six metals simultaneously in various matrices at concentration levels down to 10xe2x88x9210 M. The best analytical results have been obtained using an in situ plated mercury film (T. M. Florence, J. Electroanal. Chem. 27(1970)273); however this approach, like pre-plated mercury films, can result in unacceptable toxicity and other environmental considerations.
Because of the toxicity of mercury, considerable efforts have been devoted to the investigation of alternate electrode materials (E. P. Achterberg, C. Braungradt, supra: J. Wang and B. Tian, Anal. Chem. 65(1993)1529; M. A. Nolan, S. P. Kounaves, Anal. Chem. 71(1999)3567). While a wide range of non-mercury electrodes, including gold, carbon, and iridium, have been examined, the overall performance of these alternative stripping electrodes has not approached that of mercury ones.
U.S. Pat. No. 3,855,099 discloses an electrode, such as a graphite electrode, for anode stripping voltammetry in which there is a metal on the surface of the electrode. Among the metals specifically taught are bismuth and a bismuth amalgam. However, this patent only discloses use of bismuth for testing for phosphate ions, which is not a metal, and specifically states that the metal for the electrode surface must be more xe2x80x9cnoblexe2x80x9d than the analyte element, which is to say that the metal for the electrode surface should be below the analyte element in the electromotive series. Thus U.S. Pat. No. 3,855,099 discloses only detection of a nonmetallic analyte element, phosphate ion, and does not disclose use of bismuth in the analyte solution.
U.S. Pat. No. 5,830,343 relates to electrochemical analysis using metals deposited on electrodes, and teaches detection of bismuth, but does not teach use of bismuth in the electrode coating. U.S. Pat. No. 5,460,710 teaches use of metallic electrodes, including a bismuth electrode for use in pH titrations, but does not teach use of a carbonaceous electrode coated with bismuth for detection of a metal analyte by means of electrochemical stripping analysis. U.S. Pat. No. 5,391,270 teaches a method for detection of heavy metals, including bismuth, by means of forming soluble metallic complexes by use of an iodine/iodide solution and thereafter voltammetrically determining the peak stripping current value for the metallic complex in the iodine/iodide solution.
U.S. Pat. No. 5,292,423, to the inventor herein, and U.S. Pat. No. 5,635,054 each teach methods and devices for trace metal testing using mercury-coated screen printed electrodes, and electrodes coated with other metals. However, use of bismuth-coated screen printed electrodes is not specifically disclosed in either patent.
There is a need for a non-mercury electrode that offers high-quality stripping performance similar to that of mercury electrodes, and preferably a coated electrode that permits simple in situ preparation, high sensitivity, a well defined and undistorted stripping signal, and excellent resolution, particularly of neighboring peaks. Most critically, there is a need for an electrode that is more xe2x80x9cenvironmentally-friendlyxe2x80x9d and that exhibits very low toxicity, with a performance at least approaching that of mercury.
The invention provides a method for analyzing metals in a solution sample, which method includes the steps of providing an electrode, electrolytically depositing a bismuth metal film on the electrode and analyzing the solution sample for metal content using electrochemical stripping analysis with the electrode. In this method, a source of bismuth may be added to the solution sample, and the bismuth metal film electrolytically deposited on the electrode in the presence of the solution sample. In an alternative embodiment, the bismuth metal film is electrolytically deposited on the electrode outside the presence of the solution sample.
The electode can be a carbon-containing electrode, and may be a glassy-carbon disk electrode, carbon-fiber microelectrode, thin-film electrode or thick-film electrode. The electrode may also be a screen-printed electrode.
The method can also include providing at least one reference electrode.
In the method, the step of analyzing the solution sample for metal content using electrochemical stripping analysis can include either stripping voltammetry or stripping potentiometry. In the case of stripping voltammetry, the method can include anodic stripping voltammetry or adsorptive stripping voltammetry.
In the method, the step of analyzing the solution sample for metal content can include use of the bismuth as an internal standard. The method can further include analyzing the solution sample for metal content to make a quantitative determination of the quantities of trace metal. Analyzing the solution sample for metal content can also include simultaneously analyzing the solution sample for more than one metal. The solution sample can be a sample of body fluid, and can also be any other fluid sample.
The invention also includes an apparatus for detection of trace metals in solution, which apparatus includes a plurality of thick-film electrodes on a substrate and a bismuth metal film on at least one of the electrodes. The apparatus can further include at least one reference electrode, including a silver-containing reference electrode on the substrate. The thick-film electrode can include carbon, and may be a screen-printed electrode. The apparatus can further include an electrochemical analyzer.
A primary object of the present invention is to provide a bismuth-based electrode for electrochemical stripping analysis.
Another object of the invention is to provide an apparatus and method whereby an electrode may be coated in situ with bismuth.
Another object of the invention is to provide a bismuth-coated electrode detection system for detection of metals, including but not limited to lead, cadmium, zinc, copper, indium, thallium and nickel.
Another object of the invention is to provide a bismuth-coated electrode detection system for quantitative detection and determination of metals.
Another object of the invention is to provide a bismuth-coated screen-printed electrode.
Yet another object of the invention is to provide a bismuth-coated electrode for simultaneous detection and analysis of more than one metal.
A primary advantage of the present invention is the decreased toxicity and improved environmental safety using bismuth-based electrodes as compared to mercury-based electrodes.
Another advantage of the invention is the low cost of bismuth-based electrodes for metal detection.
Another advantage of the invention is that bismuth-based electrodes may be safely disposed of without adverse environmental considerations.
A further advantage of the invention is the highly stable and reproducible response of bismuth-based electrodes.